1. Field of the Invention
The invention pertains to a two-part polyurea-urethane adhesive composition wherein the Part A prepolymer component comprises hydroxyl compounds, isocyanates and filler materials, and the Part B curative component comprises hydroxyl compounds, amine compounds and filler materials. The two-part polyurea-urethane adhesive is applied in processes for bonding engineered structures.
2. The Related Art
Two part polyurethane adhesives are used extensively in the transportation and assembly markets for bonding various composite and metallic components. They have found significant advantage over mechanical fasteners due to their ability to transfer loads over broad ranges of the bond line which in most applications translates to extended fatigue life.
A disadvantage associated with a polymeric adhesive in certain applications, particularly in the transportation and assembly markets, is that the modulus or stiffness changes across the in-service temperature range from −35° C. to 85° C. The polymer may become overly hard and brittle at sub ambient temperatures and overly soft at elevated temperature. This observation is exaggerated with a typical phase separated polyurethane adhesive that segregates to both hard and soft domains. These domains typically show up as discrete glass transition temperatures (“Tg”), one at sub-ambient temperature representing the soft phase and the other at elevated temperature representing the hard phase. The disadvantage of the two phase morphology is the stiffness of the adhesive rapidly transitions from a firm load bearing polymer to a soft leathery material as it transitions through its high temperature Tg leading to inconsistent bulk properties.
Two component polyurea-urethane adhesives are known in the art. For example, such adhesives may be obtained by the reaction of a polyol mixture, an aromatic diamine and an isocyanate terminated prepolymer to extend the open time, maintain fast reaction and create a nearly 1:1 ratio for use in fast reaction injection molding applications. Also known are polyurea-urethane adhesive based on a slow reacting uretonimine modified isocyanates, two part structural primerless urethane adhesives for bonding polyester composites comprising a curative having a mixture of low molecular weight polyether polyol with both slow reacting and fast reacting amines, a two part polyurea-urethane adhesive for bonding elastomers having a curative comprising a mixture of a polymer of di-p-aminobenzoate containing a poly(tetramethyleneglycol) backbone with molecular weight ranging from 1,000 to 2,900 and an aromatic diamine and long open time two part polyurethane adhesives comprising a diisocyanate in one part and a curative consisting of a mixture of polyols with molecular weight varying from 2,000 to 12,000 bearing secondary hydroxyls blended with either short chain secondary polyols with molecular weight ranging from 90 to 800 with functionality greater than 2 or short chain secondary polyol with a similar short chain polyol with functionality equal to 2.
It would be commercially advantageous to provide a polyurethane adhesive that would combine characteristic advantages of good strength, low sag, high tensile strength and Young's modulus, adhesion and fast cure with a single Tg product that demonstrates improved stiffness at elevated temperature. Increased elongation is also desirable, particularly with improved stiffness at elevated temperature. Adhesive having these properties would be highly desired, such as in the transportation and assembly markets for bonding various composite and metallic components.